A hard urethane foam is formed in such a manner that a urethane prepolymer solution is injected into a lower die included in a mold that also includes an upper die, the mold is closed, the urethane prepolymer solution is foamed and then cured, the mold is opened, and the foam is then removed from the mold.
A mold for foam molding includes a lower die and an ejector pin attached to the lower die. When a molding is removed from the blow mold, the molding is pushed up with the ejector pin; hence, the molding is usually bruised with the ejector pin.
As an example of a method of efficiently removing a molding from a die without using any ejector pin, Japanese Unexamined Patent Application Publication No. 9-234748 discloses a foam molding method in which a film is provided on a cavity face of a lower die and a molding is removed from a mold after mold opening in such a manner that air is forced between the film and the cavity face such that the film is separated from the cavity face and the molding is forced out of the lower die by the film.
FIGS. 9A to 9C are sectional views illustrating this method and the mold therefor. This mold includes an upper die 2 and a lower die 3. The upper and lower dies 2 and 3 are detachably coupled to each other. The lower die 3 has a cavity 4.
A polypropylene film 6, formed by a vacuum molding process, having a shape that is similar to or the same as that of the cavity 4 is disposed on the lower die 3. The film 6 is fixed on an upper end face of the lower die 3 with fixing pins 7. Furthermore, the film 6 is clamped between packings 9, disposed on the upper end face of the lower die 3, for film-air sealing and film retainers 8 such that the film 6 is securely fixed to the lower die 3.
An air chamber 10 with a box shape is disposed under the lower die 3. One end of an air tube 12 equipped with a pressure control valve 11 is connected to the air chamber 10 and the other end is connected to an air supply/exhaust device 13. The air chamber 10 communicates with the cavity 4 through a plurality of air perforations (air holes) 14.
The film 6 is vacuum-formed by making use of the lower die 3. That is, the film 6 made of polypropylene is formed so as to have a shape similar to that of the cavity in such a manner that four sides of the film 6 are clamped, the upper and lower faces of the film 6 are heated at 180-200° C. for 15-20 seconds with a heater, the heated film 6 is fixed on the lower die 3 as described above, the cavity 4 and the air chamber 10 disposed under the lower die 3 are evacuated to a predetermined pressure by operating the air supply/exhaust device 13, and the film 6 is thereby pressed against the lower die.
In order to manufacture a hard polyurethane foam molding using the mold 1, a predetermined amount of a source material for hard polyurethane foams is placed on the film 6 disposed in the cavity 4 and the upper die 2 is then fitted to the lower die 3. After the source material is foamed and then expanded as shown in FIG. 9A, the upper die is opened. As shown in FIG. 9B, the air supply/exhaust device 13 is operated; air is introduced into a gap 15 between the film 6 and the cavity 4 through the air tube 12, the air chamber 10, and the air perforations 14; and a molding 16 is pushed up together with the film 6, whereby the molding 16 is removed.
Air is then evacuated with the air supply/exhaust device 13, whereby the film 6 is brought into close contact with the cavity face as shown in FIG. 9C in preparation for the next cycle.
The method disclosed in Japanese Unexamined Patent Application Publication No. 9-234748 requires a step of bringing the film 6 into close contact with the lower die 3 after demolding and the step requires time and manpower. Furthermore, if the film 6 is wrinkled, the film wrinkles are probably transferred to the molding.